1. Field of the Invention
The present invention generally relates to an image formation apparatus using electro photography, such as copying machines, printers, facsimile apparatuses, and compound machines; and a fixing apparatus thereof. Further, the present invention especially relates to a fixing apparatus and an image formation apparatus that are capable of forming an image on two or more kinds of recording media that have different dimensions.
2. Description of the Related Art
A fixing apparatus using an electromagnetic-induction heating (IH) method has been known, which aims at saving energy by shortening the standup time of an image formation apparatus, such as a copying machine and a printer (for example, Patent Reference 1).
According to Patent Reference 1, for example, the fixing apparatus of an electromagnetic-induction heating method includes a fixing belt, serving as a fixing member, that is installed with tension, and supported by a heating roller and a fixing roller, an electromagnetic-induction heating unit (IH unit) that is installed countering the heating roller with the fixing belt in-between, and a pressurizing roller that is installed countering the fixing roller with the fixing belt in-between. Further, the IH unit includes a coil unit and a core that is installed countering the coil unit, the coil unit being installed in the directions of the width (the directions that perpendicularly intersect the conveyance direction of the recording medium).
Here, the fixing belt is heated at a position that counters the IH unit. The heated fixing belt provides heat to a toner image on the recording medium conveyed to the position of the fixing roller and the pressurizing roller such that the toner image is fixed. Specifically, a high frequency alternating current is provided to the coil unit, which generates a magnetic field around the coil unit, causing an eddy current to arise on the surface of the heating roller. The eddy current in the heating roller generates the Joule heat with the electric resistance of the heating roller. The Joule heat raises the temperature of the fixing belt that is installed around the heating roller.
It is known that the IH heating method raises the temperature of the surface of the fixing belt (fixing temperature) to a desired temperature with a small amount of energy and in a short time.
Further, Patent Reference 2, for example, discloses a technology of dividing an IH coil (coil unit) of the fixing member into two or more sections in the width directions, aiming at preventing the temperature from rising at places where the recording medium does not pass, when the width of the recording medium is small. According to this technology, two or more IH coils are installed in the width directions, and power is supplied only to one or more selected IH coils such that the heating range in the width directions is optimized.
Further, Patent Reference 3, for example, discloses a fixing apparatus that uses the IH method, wherein two or more core units are arranged in the width directions in the fixing member (heating member), aiming at preventing the temperature from rising at places where the recording medium does not pass when the width of the recording medium is small. According to this technology, a bimetal member used such that, based on the temperature distribution of the fixing member in the width directions, core units near a high temperature part are moved far from the fixing member. In this way, a local temperature rise of the fixing member in the width directions is prevented from occurring.
Further, Patent Reference 4, for example, proposes a fixing apparatus using the IH method, and including a shielding member (a magnetic-flux shielding plate) aiming at preventing the temperature rise at places of the fixing member (heating medium) where the recording medium does not pass when the width of the recording medium is small, the shielding member having a circle-curved surface. According to this technology, the shielding member is rotated such that the heating range in the width directions is made the same as the width of the recording medium, and the temperature at both ends in the width directions of the fixing member is prevented from rising.
[Patent reference 1] JPA, 2002-82549
[Patent reference 2] JPA, 2001-34097
[Patent reference 3] JPA, 11-109774
[Patent reference 4] JPA, 2002-352948
[Problem(s) to be Solved by the Invention]
Image formation apparatuses having the conventional fixing member as described above have various problems caused by preventing the temperature rise at both ends in the width directions when fixing a recording medium having a small width.
The problems are specifically described as follows.
Common image formation apparatuses are capable of performing an image formation on several kinds of recording media with different widths. Here, the widths include JIS sizes, such as A-sequence and B-sequence, and non-standard widths. Even in the case of the standard sizes, the recording media may be conveyed in the direction of the longitude and in the perpendicular direction thereof, which constitute different widths.
When the fixing apparatus processes the recording media having different widths, heat distribution of the fixing member in the width directions becomes uneven depending on the widths of the recording media. When a recording medium having a small width is fixed, heat is consumed over the width of the recording medium (paper passing range, central position), and the temperature of the paper passing range is lowered as compared with other ranges (non paper passing range, end positions). This phenomenon becomes especially remarkable when the small width recording media are continuously processed.
While controlling the temperature of the central position of the fixing member in the width directions is possible, it is difficult to control the temperature on both ends of the fixing member, where the temperature undesirably rises. Especially, with the fixing member using the IH method, the temperature of the fixing apparatus quickly rises. When the heat at both ends of the fixing member is not transferred, the temperature quickly rises. If, under this circumstance, a recording medium having a great width is fixed, “hot offset” will occur toward both ends in the width directions of the recording medium. Further, if the temperature of both ends exceeds a thermal breakage temperature, it is conceivable that the fixing member will be thermally damaged, and the service life of the fixing member will be shortened.
In order to solve the problem as described above, Patent Reference 2 provides two or more electromagnetic-induction coils arranged in the width directions, and a heating range of the fixing member is adjusted in accordance with the width of a recording medium.
According to this technology, however, electromagnetic-induction coils that are of a complicated structure have to be prepared, and have to be individually controlled in accordance with various widths of the recording media. Due to the complicated structure and control, the solution tends to be costly. Further, since there arise gaps between the electromagnetic induction coils, the temperature of the heating member at positions corresponding to the gaps tends to be lower than required, causing poor fixing at the positions.
According to a solution proposed by Patent Reference 3, for example, two or more core units are installed in the width directions, and a bimetal member is provided to the core units located at each end of the width. The bimetal member causes the core units located at each end of the width to separate from the fixing member if the temperature at the end positions rises because there is no recording medium.
This technology also requires two or more core units corresponding to various widths of the recording media, and the core units have to be individually controlled by the bimetal member. Accordingly, the structure and control tend to be complicated, and cost tends to be high. Further, there arise gaps between the core units. Positions on the heating member corresponding to the gaps cannot be properly heated, causing poor fixing at the positions.
In order to solve the problems arising from providing two or more electromagnetic-induction coils and core units in the width directions, a method is conceived wherein magnetic flux at both ends in the width directions is shielded by a shielding member. Namely, Patent Reference 4, for example, discloses a technology wherein a shielded range is made variable according to the width of the recording medium such that the range wherein the magnetic flux is shielded at both ends of the width is adjustable. In this manner, it is expected that even heating of the fixing member will be available in accordance with the width of the recording media.
However, even in this case, when the shielding of the core unit by the shielding member is completely open (i.e., heating range is maximized), the temperature of both ends of the fixing member in the width directions is lowered. For this reason, when a recording medium with a great width is to be fixed, poor fixing tends to occur at both ends of the recording medium in the width directions.
Further, when the heating range is made variable by varying the shielded range at both ends in the width directions according to the width of the recording media, as the technology of Patent Reference 4, the heating range tends to fluctuate. If the heating range of the fixing member in the width directions fluctuates, “hot offset” and “cold offset” occur at both ends of the recording medium in the width directions after a fixing process. Specifically, when the heating range is greater than desired, the temperature at both ends of the fixing member in the width directions is raised, and “hot offset” occurs. If the heating range is smaller than desired, “cold offset” occurs.